Messages in this thread Patch in this message | | | From | "Yandong.Zhao" <> | Subject | [PATCH] arm64: neon: Add preemption protection for kernel_neon_busy | Date | Mon, 9 Jul 2018 20:19:43 +0800 |
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From: Yandong Zhao <yandong77520@gmail.com>
Dear Dave,
The scenario for this bug is: The A process is sched out when the CPU0 executes the function raw_cpu_read(kernel_neon_busy) and just gets the address of kernel_neon_busy without reading. The B process starts running kernel_neon_begin() on CPU0, and the variable kernel_neon_busy on CPU0 becomes true. At this time, the A process is executed on CPU1 and the kernel_neon_busy value is CPU0 (true),so BUG_ON()!
crash64> kernel_neon_busy PER-CPU DATA TYPE: bool kernel_neon_busy; PER-CPU ADDRESSES: [0]: ffffffc07fee30a0 [1]: ffffffc07fef90a0 [2]: ffffffc07ff0f0a0 [3]: ffffffc07ff250a0
CPU0 CPU1 | | A task have get addr ffffffc07fee30a0 | and sched out | | | B task kernel_neon_begin() | [ffffffc07fee30a0]=1 | | | | A task sched in and read | [ffffffc07fee30a0]==1,so BUG_ON. | | B task kernel_neon_end() | [ffffffc07fee30a0]=0 | | |
Signed-off-by: Yandong Zhao <yandong77520@gmail.com> --- arch/arm64/include/asm/simd.h | 16 ++++------------ 1 file changed, 4 insertions(+), 12 deletions(-)
diff --git a/arch/arm64/include/asm/simd.h b/arch/arm64/include/asm/simd.h index fa8b3fe..6580dcd 100644 --- a/arch/arm64/include/asm/simd.h +++ b/arch/arm64/include/asm/simd.h @@ -29,20 +29,12 @@ static __must_check inline bool may_use_simd(void) { /* - * The raw_cpu_read() is racy if called with preemption enabled. - * This is not a bug: kernel_neon_busy is only set when - * preemption is disabled, so we cannot migrate to another CPU - * while it is set, nor can we migrate to a CPU where it is set. - * So, if we find it clear on some CPU then we're guaranteed to - * find it clear on any CPU we could migrate to. - * - * If we are in between kernel_neon_begin()...kernel_neon_end(), - * the flag will be set, but preemption is also disabled, so we - * can't migrate to another CPU and spuriously see it become - * false. + * Operations for contexts where we do not want to do any checks for + * preemptions. Unless strictly necessary, always use this_cpu_*() + * instead. */ return !in_irq() && !irqs_disabled() && !in_nmi() && - !raw_cpu_read(kernel_neon_busy); + !this_cpu_read(kernel_neon_busy); } #else /* ! CONFIG_KERNEL_MODE_NEON */ -- 1.9.1
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